Geochemistry of lead in modern seafloor hydrothermal systems

Polymetallic sulphide mineralization occurs in various modern seafloor tectonic settings in association with hydrothermal systems. As an ore forming metal, Pb is an integral component of these deposits. Leaching of the volcanic pile and direct contributions from the parent magma are the two possible mechanisms leading to the concentration of Pb in hydrothermal deposits.; Pb concentrations in melt inclusions from 9 study sites indicate enrichment in back-arc basin magmas with respect to magmas from other, non arc tectonic settings. Basalt-hosted melt inclusions from the Okinawa back-arc have >1.5 ppm Pb (average of 3.1 ppm, n=13) while basalt-hosted inclusions from all other non-back-arc sites have <1.5 ppm Pb (with some exceptions). Dacite-hosted inclusions from the Manus Basin and Bransfield back-arcs average 5.2 ppm (n=17) and 6.6 ppm (n=15) Pb respectively.; Analyses of altered volcanic material underlying the PACMANUS hydrothermal field (Manus Basin) indicate a 200 in thick Pb-enriched zone within a non-sulphide mineral assemblage. Pb concentrations in the enriched zone is >5.6 ppm, the average Pb content of fresh PACMANUS dacites, signifying that the process of leaching by exposure to hydrothermal fluids is heterogenous within the volcanic pile. Fluids may be stripping Pb from one zone and accumulating it in another reducing the overall volume of volcanic rock that can act as a source for Pb in the main polymetallic deposits.; Mass balance modeling of the PACMANUS system demonstrates that for a stationary reaction zone, unrealistically high leaching, transport and precipitation efficiencies would be needed to account for the Pb in the sulphide and non-sulphide deposits. Even when accounting for migration of the reaction zone (dynamic model) to expose larger amounts of fresh volcanic material, the required volumes of fresh rock remain high.; Modeling of the availability of Pb in the PACMANUS magmatic source, and of the ability of this Pb to be transferred effectively into the hydrothermal system through pre-eruptive degassing, demonstrates the viability of direct magmatic contributions. Large variations in hydrothermal discharge rates and the longevity of activity have a limited effect on the overall potential for a degassed magmatic component to be the main metal source for the Pb-rich deposits.